An extension is a software package that extends your Joomla! installation in some way. A small selection of extensions is included with the default Joomla! installation but many more are available from the Joomla! Extensions Directory.

The term extension is generic and the following specific extension types are available (listed alphabetically):
Component (since ) – adds custom functions to your site that can be selected from menus
Language (since ) – defines an additional language for your site
Library (since ) – provides functions to be used by other extensions
Module (since ) – shows nonessential data in a side box, possibly on multiple pages
Package (since ) – bundles related extensions
Plugin (since ) – modifies content in articles or provides functions to extend other extensions
Template (since ) – define the look, feel, and navigation capabilities of your site

For an overview of the most important extension types and the functionality they provide, see Extension types (general definitions)

Reference: http://docs.joomla.org/Glossary

The Joomla site that is accessed by visitors without user privileges. Front page refers to the public home page of your Joomla site.

All critical settings of a Joomla site and server. They include the following: Site, System, Server, Permissions, and Text filters. You can access the Global Configuration settings through the Site menu in the Administration control panel.

Acronym for Free or open-source software. Software, like Joomla, that is available to use, copy, study, and change in any way without charge. FOSS software typically uses a special software license, such as GPL, that requires any derivative works to also be free.

A block of PHP code that has a name and it has a property that it is reusable it can be executed from as many different points in a PHP Program as required. Also known as a method (if it is inside a class). Most Joomla code is inside functions/methods.

Stands for General public license. The most widespread such license is the GNU General Public License, GNU GPL or GPL for short. This software license is available in differrent versions and used by Joomla and many other FOSS projects. Joomla is licensed under GPL2 now.